You sort of have two choices
What is your window size / view-port size resolution. ?
in project properties
(also leave sampling to off/point for this)
Use a high resolution for your Window Size / Viewport size in project properties and that will be the resolution for your artwork in game
enable "high quality" under Full screen scaling
this will display all artwork at the highest possible resolution regardless of window size (window size will only be used for aspect ratio and movement calcs etc)
I went into a bit more detail here.